A common drive unit for work vehicles, such as skid steer loaders, is a hydrostatic drive unit. Hydrostatic drive units are advantageous because they are capable of providing a range of different speeds without the need for mechanical gearing assemblies. Typically, hydrostatic drive units include a hydraulic pump or pumps which are powered by the engine of the work vehicle. The engine may be, for example, a conventional diesel or gasoline engine having a flywheel which turns the pump. Thus, the pump speed and the corresponding fluid flow rate are directly proportional to the engine speed.
In most instances, the hydraulic pump has a displacement chamber for hydraulic fluid. The fluid flow is controlled by the angular position of a swashplate the pump, which controls the effective length of the displacement chamber. The effective length a the displacement chamber, in turn, regulates the hydraulic fluid flow produced by the pump. As is generally understood, the pump may be fluidly connected to hydraulic devices, such as hydraulic cylinders, for driving various actuators, such as lift arms or ground stabilizers, attached to the work vehicle. In addition, the hydraulic pump may be fluidly connected to a drive motor. The fluid flow from the pump causes the drive motor to rotate the axles, which drive the wheels and, thus, the work vehicle. Typically, a separate motor is provided for the right-side and left-side wheels of a work vehicle, with each motor being fluidly connected to a separate hydraulic pump. As such, the speed of the right-side and left-side wheels may he independently controlled for improved performance of the work vehicle.
Typically, a hydrostatic drive unit is controlled by either a closed-loop control system or an open-loop control system. With closed-loop control systems, the system automatically maintains the displacement of the pump at a fixed swashplate position (that is proportional to the control input provided by the operator) regardless of the loading condition of the work vehicle. As such, closed-loop control systems typically provide enhance vehicle controllability and performance. However, due to the automatic correction of the swashplate position, closed-loop control systems fail to provide the operator with any operational feedback (e.g., reduced vehicle speed) as loads on the vehicle increase. In contrast, open-loop control systems are load-sensitive and, thus, provide the desired “feeling” of heavy and/or light loads. For example, open-loop control systems are often designed such that, when loads on the vehicle increase, the swashplate automatically de-strokes, thereby providing the operator a tactile indication of the increased loads (i.e., due to the reduction in vehicle speed). However, depending on the operational efficiency of the pump, such de-stroking of the swashplate typically results in a substantial loss in the vehicle's productivity and/or controllability.
Accordingly, a system and method for controlling a hydrostatic drive unit of a work vehicle that can provide the advantages of both closed- and open-loop control systems would be welcomed in the technology.